1. Field of the Invention
The present invention relates to headboxes used in forming a paper web in general, and to headboxes employing adjustment mechanisms in particular.
2. Background of the Invention
Paper is made of individual fibers which are deposited in a continuous sheet. The sheet is typically formed from a papermaking stock consisting of less than 1 percent wood fibers dispersed in more than 99 percent water. The fibers and water are deposited onto a forming fabric, or between two forming fabrics, in the former section of the paper machine to form a continuous web of paper. The papermaking stock is first fed to a headbox which distributes the stock across the width of the forming fabric or fabrics on which the paper web is being formed. The headbox discharges the stock through a long narrow converging nozzle or slice which injects the stock onto the rapidly moving wire screen or between two screens. The fibers are retained on the fabric or wire surface while the majority of the water is drawn through the fabric or fabrics. The former may be a single wire horizontal former (Fourdrinier) or a two wire (twin wire) former. The paper web thus formed is pressed, dried and wound into reels. The reels of paper formed on the papermaking machine are then further processed to produce smaller rolls or sets of paper for printing. Individual sheets are also made which may be used in sheet-fed printing presses, in copy machines and in laser printers.
Because paper is made of individual paper fibers which are joined together during the pressing and drying process, the orientation of the fibers within the paper controls the physical properties of the paper. In particular, fiber orientation influences the strength and dimensional stability of the paper. It has been found that paper, when exposed to heat or moisture, will form more wrinkles or become wavy when the fibers are insufficiently uniform in fiber orientation. Exposing paper to heat or moisture causes the paper to shrink or expand. It is the nonuniformity of the dimensional changes which causes the paper to wrinkle or cockle. Nonuniformities in the paper are in turn caused by fiber alignment variations.
Printing presses, converting equipment and papermaking machines are increasing in speed. This means they are more sensitive to small instabilities in the paper web such as those caused by nonuniform dimensional changes in the paper. The instabilities can lead to web breaks or print quality problems. The printing industry in newspapers, magazines and books continues to use more and more color which results in more water or other liquids coming in contact with the paper web where they can release dried-in stresses which bring out the dimensional instability of the paper and cause it to wrinkle. At the same time, increased moisture decreases the paper strength making it more subject to breaking.
Further, the consuming public has come to expect not only more color printing but printing of higher quality. Slight cockling or warping of the paper can lead to unprinted areas. Where glossy paper is utilized, waviness or cockle results in nonuniform reflection which is distracting to the consumer.
The fact that a sheet or web of paper can become wavy upon exposure to moisture or heat has thus become of greater concern. Most processes which form an image upon paper employ heat or moisture. When paper in sheet form is processed through a photocopier, laser printer or printing press, warping of the sheet may cause it to jam the machine and cause a significant loss of productive time. When paper in the form of a continuous web becomes wrinkled, it is liable to break. Breakage of a web within a printing press, in a winder or on a coater causes downtime, as well as a significant loss of paper.
The problem of dimensional changes in finished paper is aggravated by the trend to use lower base weight paper to hold down paper costs. Lighter-grade papers are more subject to press breakage or jamming. A lighter grade of paper also means that for a given amount of moisture transferred by printing, particularly of colored images, a greater percentage of moisture is introduced into the paper. The increased productivity of modern equipment means that even limited downtime to clear a jam or rethread a broken web can have significant economic consequences in terms of lost production. Further, paper must lie flat for easier handling, loading and compact transportation.
By the time the paper web leaves the former, the orientation of the fibers is fixed. Influencing fiber orientation of the web after the headbox in the forming section is difficult and largely impractical. Thus if greater uniformity of fiber orientation is to be achieved, the adjustment must be accomplished in the headbox. Thus the headbox must not only avoid introducing undesirable flow variations, it must attempt to correct for problems arising in the former.
Various means for controlling flow and scale of the turbulence produced in a headbox between the stock input header and the slice gap or opening are known. One known type of headbox employs a bank of parallel tubes which employs small scale turbulence generators and pressure drop features to assure a more uniform flow of stock into the nozzle and from the slice opening onto the forming wire.
Various means have been employed to enable adjustments to be made to the tube bank flow, including chamfering the tubes or installing inserts in specified tubes. However, these adjusting means are shutdown modifications, requiring downtime which can have significant economic consequences in terms of lost production, and are often viewed merely as temporary methods, with no long-term significance.
These approaches are also susceptible to headbox cleanliness problems, such as clogging, and do not take into account other more dynamic factors that affect fiber orientation resulting from headbox cross flow tendencies, such as headbox header balance, slice lip profile and use of pond-side bleeds. In addition, such modifications increase the significance of predicting the optimum edge flow relationship during the design stage for each headbox, when exact operating details are unknown. U.S. Pat. No. 5,470,439 to Makino et al. shows a system of valves for adjusting the flow of stock from a headbox along the pond sides. The valves allow adjustments of the flow of stock by a flow rate controller which receives data such as the lip opening degree, wire velocity, cross directional basis weight profile data and the like. However, Makino et al. only allows adjustment of the flow at the ends of the headbox.
What is needed is a headbox which deposits a fiber mat of more uniform fiber orientation onto a forming fabric.